1. Field of the Invention
The present invention relates to a class of broadly tunable single-mode current-injection semiconductor laser sources
2. Brief Description Of The Related Art
Quantum Cascade Lasers (QCLs) are semiconductor lasers that are unipolar and can work in Mid-Infrared and Terahertz spectral regions, which are very important for chemical and biological sensing, remote sensing, high-resolution spectroscopy, infrared detection, countermeasures, and many other applications. In QCLs, optical transitions occur between confined electronic sub-bands of a semiconductor heterostructure. As a result, the emitted photon energy is determined by the thicknesses of the wells and barriers in a heterostructure and can be tailored by band gap engineering. This makes possible fabricating QCLs that emit simultaneously at two or more widely separated wavelengths (this can be done, for example, using heterogeneous active region, consisting of a stack of two or more active regions designed for emission at specific wavelengths, see, for example, a report on QCL emitting at wavelengths 5 and 8 microns by C. Gmachl, D. L. Sivco, J. N. Baillargeon, A. L. Hutchinson, F. Capasso, A. Y. Cho, Applied Physics Letters v. 79, p. 572 (2001)) and fabricating broadband-gain QCLs (this can be done using an active region of “bound-to-continuum” design, see, for example, R. Maulini, M. Beck, J. Faist, E. Gini, Applied Physics Letters v. 84, p. 1659 (2004), using an active region that comprises a stack of two or more active regions based on bound-to-continuum design, each designed for an emission at different wavelengths, see, for example, R. Maulini, A. Mohan, M. Giovannini, J. Faist, E. Gini, Applied Physics Letters v. 88, 201113 (2006), or using heterogeneous active region, consisting of many active regions designed for emission at slightly different wavelengths, see, for example, C. Gmachl, D. L. Sivco, R. Colombelli, F. Capasso, A. Y. Cho, Nature v. 415, p. 883 (2002)). Thus, a single QCL chip can emit light in wide ranges of mid-IR frequencies. We note that, similarly, one can also design QCLs that emit light in the wide range of Terahertz frequencies (see, for example, Benjamin S. Williams, Sushil Kumar, Qing Hu, and John L. Reno, Optics Letters v. 30, p. 2909 (2005)).
Single mode emission is required for most of the applications. To enforce single-mode emission, QCLs are either processed into distributed feedback (DFB) lasers (see, for example, Jerome Faist, Claire Gmachl, Federico Capasso, Carlo Sirtori, Deborah L. Sivco, James N. Baillargeon, and Alfred Y. Cho Applied Physics Letters v. 70, p. 2670 (1997)) or used in external cavity tunable lasers configuration (see, for example, G. P. Luo, C. Peng, H. Q. Le, S. S. Pei, W.-Y. Hwang, B. Ishaug, J. Um, James N. Baillargeon, and C.-H. Lin, Applied Physics Letters v. 78, p. 2834 (2001) and R. Maulini, A. Mohan, M. Giovannini, J. Faist, E. Gini, Applied Physics Letters v. 88, 201113 (2006)). External cavity QCLs have wide tunability but are cumbersome and complex to build as they require well-aligned external optical components and a grating for tuning. DFB lasers are very compact, but DFB QCLs to date have limited tunability, which is achieved either by changing the temperature of the device (see, for example, C. Gmachl, D. L. Sivco, J. N. Baillargeon, A. L. Hutchinson, F. Capasso, A. Y. Cho, Applied Physics Letters v. 79, p. 572 (2001)) or by changing the bias and/or current through the device (see, for example, C. Gmachl, F. Capasso, A. Tredicucci, D. L. Sivco, J. N. Baillargeon, A. L. Hutchinson, and A. Y. Cho, Optics Letters v. 25, p. 230 (2000)).